katedralo

A cathedral is not a big church. It is a bishop's church. The distinction is in the etymology and it is the one fact about the word worth carrying: cathedra is a seat, the katedralo is the building that holds the bishop's throne, and size has nothing to do with it. There are cathedrals smaller than the parish church down the road, and there are enormous churches - Westminster Abbey among them - that are not cathedrals at all because no bishop sits in them. Get that right and you know more than most native English speakers do about their own word.

The ladder of buildings, from generic to specific.Preĝejo, at rank 528, is the general Esperanto word: a pray-place, preĝi plus -ej-, and it covers any building where people worship, of any faith. Templo is a temple, and it is what the language reaches for outside the Christian frame. Kapelo is a chapel, baziliko a basilica, abatejo an abbey. Katedralo sits at the specific end, tied to an episkopo and a diocese - episkopejo for the see itself. And because Esperanto builds rather than borrows wherever it can, ĉefpreĝejo, head-church, turns up as a native alternative to katedralo and is perfectly good Esperanto.

Esperanto is officially neutral about all of this, which is more interesting than it sounds. The UEA's member list carries a Catholic association and an atheist one side by side, and the language supplies the vocabulary for both without taking a position - Esperanto kaj religioj works through how that neutrality is maintained in practice. Kirko also exists as a rarer synonym for preĝejo, borrowed from the Germanic side, but preĝejo is the word the course teaches and the word you should use.